Process of corona treating a thermoplastic tubular film

ABSTRACT

A meat product package including an enclosing film having an EVA-containing inside surface and an in situ aqueous medium-cooked meat product in adhering relation to the film inside surface as the meat contacting and adhering surface. Starch particles are preferably dispersed across the meat contacting surface which has been both irradiated and subjected to corona treatment. A method for corona treating a thermoplastic tube inside surface in which small particles within the flat tube separate opposite surfaces providing voids, and the electric discharge crosses the flat tube through the voids.

This application is a division of prior U.S. application Ser. No.07/872,873 filed Apr. 23, 1992 which is now U.S. Pat. No. 5,328,705issued Jul. 12, 1994.

TECHNICAL FIELD

The present invention relates to pre-cooked food products and moreparticularly to food products such as meat packaged and cooked in a filmwherein the film reduces exudation of liquids from the food productduring cooking. Another aspect of the invention relates to a method forcorona treating the inside surface of a flexible thermoplastic film.

BACKGROUND OF THE INVENTION

Food products which are packaged in so-called "cook-in" films are wellknown in the art. Cook-in films, either in the form of tubular casingsor bags, are used for packaging a food product such as meat wherein thefood product is enclosed in the film and then processed, eg. cooked, insitu usually by immersion in a hot water bath. Typical food productspackaged and processed in this fashion include, among others, hams,turkey hams, fish and poultry rolls.

Food products packaged and processed in this fashion are often sold tothe retail consumer market still encased in the film. As an alternative,the processor has the option to remove the film after cooking andrepackage the cooked food product either whole or sliced, for retailsale. In either case, a very desirable feature of the food productpackage is that the cook-in film suppresses the formation of "cook-out",also referred to as "purge". The terms "cook-out" and "purge" refer tothe liquid which tends to exude from a food product during cooking. Thisliquid exudate generally comprises water, water-fat emulsion, broth orother food juices. Cook-out is objectionable for several reasons. Forexample, it will cause a layer of liquid to form between the film andfood, and also collect as pools in any voids between the film and thesurface of the food product. In cases where the cook-in film is notremoved for retail sale, these layers or pockets of exudate liquiddetract from the appearance of the package and may deter consumerpurchase. Also, this accumulation of liquid adversely affects thepreservability of the cooked, packaged food.

The exudate liquid further represents an undesirable weight loss. Thisis especially true in cases where the cook-in film is removed forslicing and/or repackaging, in that the liquid is not reincorporatedinto the repackaged food product.

To a large extent, the quantity of cook-out or purge is dependent uponthe ability of the surface of the food product to wet the foodcontacting surface of the packaging film. If the film surface is wettedby the food product, the film will adhere to the food product surfaceduring cooking and this adherence will prevent cook-out.

"Wetting" is defined for purposes of the present invention as anaffinity between the film surface and the food. One indication of thisaffinity is the wetting tension of the film surface as measured by ASTMD2578-67.

Using meat emulsion as an example, adherence of a film to the meatproduct surface will increase as the wetting tension of the film surfaceincreases. However, if the adhesion is too high, the meat surface isdisrupted by chunks of the cooked meat adhering to the cook-in film whenthe film is removed. In practice, the adhesion must be controlled toprovide the adhesion necessary to prevent cook-out while keeping theadhesion below a level which causes chunks of cooked meat to pull offwhen the cook-in film is removed.

A further advantage of having the cook-in film adhere to the food isthat it provides the package with a satisfactory outer appearance. Thisis especially desirable where the cook-in film is not removed prior tosale at retail.

Accordingly, a cooked food product package including a cook-in filmencasing a food product cooked in situ within the film wherein the foodproduct wets the film so the film tightly adheres to the cooked foodsurface not only improves the package appearance but also extends shelflife and reduces weight loss attributed to the liquid cook-out.

Other desirable characteristics for cook-in films include high tensilestrength and puncture resistance at typical cooking temperatures(68°-82° C.) and, for certain end uses, the film should have a lowoxygen permeability and be heat shrinkable. These desirable cook-in filmcharacteristics are preferably obtained by constructing a laminate film.For example, a meat adhering cook-in film as disclosed in U.S. Pat. No.4,784,863 is a three layer film wherein an oxygen barrier layercomprising a vinylidene chloride-methyl acrylate copolymer (MA-VDC) isdisposed between inner and outer layers. The outer layer is composed ofa material, or a blend of materials, able to withstand the abuse andabrasion of handling the package. Suitable outer layers as disclosed inthe '863 Patent include linear low density polyethylene (LLDPE), verylow density polyethylene (VLDPE), ethylene-vinyl acetate (EVA)or blendsof these materials. The inner layer, which includes the film surface indirect contact with the meat product, is an EVA having a vinyl acetate(VA) content of between about 3% and about 18%.

U.S. Pat. No. 4,888,223 (the disclosure of which is incorporated byreference) discloses a cook-in film of two or more layers. In a threelayer embodiment, the outer layer is composed of nylon. Nylon providesthe required barrier properties and is abuse and abrasion resistant. Theinnermost layer is a polyethylene. An intermediate adhesive layer bondsthe outer nylon layer and inner polyethylene layers together.

It also is known in the art to subject the cook-in film to varioustreatments to improve its food adhering characteristics. For example,U.S. Pat. No. 4,411,919 discloses that by subjecting the food adheringsurface of a polymeric olefin film to an energetic radiation in thepresence of oxygen, the surface is oxidized to render the surfacecharacteristics of the film more compatible with the surfacecharacteristics of the food product. The '919 Patent discloses thatsuitable energetic radiation treatments include corona discharge, flame,plasma, ultraviolet and electron beam radiation.

In the '863 Patent mentioned above, the food adhering property of theinner EVA layer of the cook-in film is improved by dispersing starchparticles across the food adhering film surface and then irradiating thefilm. In the above mentioned '223 Patent the food adhering property ofthe polyethylene film inner layer is increased by subjecting the foodcontacting surface to a corona treatment.

The film inner surface of Patent '863--irradiated starch containingEVA--has acceptable cook-out (purge) as measured by good meat adhesionand little "fat out" (i.e. accumulation of high fat content material inconcentrated form between the meat outer surface and the film innersurface) for most meats including premium grade boiled ham having lessthan about 10% fat and usually less than about 5% high collagen meatprotein of the total available meat protein. Unfortunately when the meatto be cooked insitu is a commodity style boiled ham having a productcomposition of more than about 10% fat and usually more than about 5%high collagen meat protein of the total available meat portion, theirradiated dispersed starch particle--containing EVA inner layer-to-meatadhesion is only marginally satisfactory. Improved cook-out (purge) asmeasured by meat adhesion for commodity style boiled ham has beenrealized with the film surface of U.S. Pat. No. 5,051,266, comprising ablend of between about 30% and about 75% of the aforementioned EVA andbetween about 25% and about 70% of an unneutralized acid copolymer of analpha-olefin. This compound has the formula RHC═CH₂ where R is H or C₁to C₈ alkyl and an alpha, beta-ethylenically unsaturated carboxylicacid, as for example ethylene acrylic acid (EAA). The film surface isirradiated at dosage of at least about 2 MR.

The Patent '266 type film surface--an irradiated EVA-EAA blend--providesimproved cook-out (purge), but based on commercial use, even furtherimprovement i.e. lower cook-out (purge) is desirable for high collagen,high fat type meats.

There is a need for an improved method for corona treating the insidesurface of a flexible thermoplastic tubular film. In the prior artmethod as for example described in the aforementioned U.S. Pat. No.4,888,223, a tube is inflated with gas in an amount at least sufficientto prevent the contact of internal surface areas of the tube. Thistransverse space corresponds to the distance between the opposing coronadischarge electrodes. Roller-type electrodes are located in transverselypositioned pairs with each pair longitudinally spaced from each adjacentpair of electrodes, with one roll member being a discharge electrode andthe other roll member being a grounded electrode.

Since the prior art roller electrodes support the longitudinally movinggas inflated tube, there is no air gap between the tube outer surfaceand the supporting roller electrode surface, and it is not possible tosimultaneously apply a significant corona discharge to the tube outerand inner surfaces, only the latter. Such simultaneous treatment may bedesirable to increase the wetting tension of the inner surface forimproved meat adhesion during cook-in, and also increase the wettingtension of the outer surface for printing thereon.

Also in the prior art system for corona treatment of inside surfaces offlexible thermoplastic tubular films, each roller electrode requires aslip ring to transfer electricity from a stationary member to therotating electrode, and these slip rings have a high wear rate. Finallythe prior art corona treatment system requires power driven nip rollsfor longitudinal movement of the gas-supported tube through thecorona-discharge region.

OBJECTS OF THE INVENTION

A primary object of one aspect of the present invention is to provide afood containing package including a cook-in film which encases an insitu, aqueous medium cooked food product wherein the inner surface ofthe film is of the EVA type and maintained in an improved wetting andadhering relationship with the food product.

Another object is to provide such a package wherein the film EVA typesurface has enhanced food adhesion properties.

A further object is to provide such a package wherein cook-out (purge)from the package, after cooking, is reduced by increasing the wettingtension of the EVA type film surface in contact with the food product.

Still another object of the present invention is to provide such apackage wherein the meat adhering EVA type layer of the cook-in film istreated to enhance its meat adhering properties.

The primary object of another aspect of the invention is to provide animproved method for corona treating the inside surface of a flexiblethermoplastic tubular film, which method does not require spaced pairsof roller-type electrodes but instead a group of fixed positionelectrodes spaced from each other in the corona discharge path.

Another object is to provide such a method which does not require a pairof mechanically driven nip rolls at each end of the corona dischargepath to drive the film tube and to seal the inflated tube whiletransversing this path.

Still another object is to provide such a corona treatment system whichis capable of simultaneously corona treating the tube outer surface andits inner surface.

SUMMARY OF THE INVENTION

One aspect of the present invention is a cooked food product packageincluding a cook-in film encasing an in situ aqueous medium cooked foodproduct. The film has a food adhering surface containing EVA, which hasbeen irradiated and treated by a corona discharge.

It is well known in the art that corona treatment will increase thewetting tension of a film and thereby improve its meat adheringcharacteristic. What is surprising, however, is that subjecting anEVA-containing film surface to both irradiation and corona treatmentwill provide a still further improvement in purge reduction and foodadherence even though there is no commensurate increase in wettingtension over a corona treated, but nonirradiated film.

In a preferred form, the film is a tube wherein the inner surface of thetubular film defines the wetting and food adhering surface. Where thefilm is in tubular form, it has been found advantageous to uniformlydisperse starch particles across the food adhering surface at a level ofabout 20 to about 30 g/1000 ft².

Preferably irradiation is at a dosage level of at least 2 MR and coronatreatment is sufficient to provide a surface wetting tension of betweenabout 36 and about 50 dyne/cm.

The tubular film has an inner surface wherein EVA may be the majorconstituent or substantially the only constituent. Alternatively theinner surface may be a blend of EVA with another compatible material asfor example a polyolefin such as ethylene acrylic acid copolymer (EAA).Although the EVA-containing surface may be the only layer, the filmpreferably comprises three layers wherein the intermediate layer is abarrier layer composed of a vinylidene chloride-methyl acrylate (MA-VDC)copolymer. The outer layer of the preferred multilayer tubular film is ablend of EVA and very low density polyethylene (VLDPE) which has adensity of about 0.912 g/cm³.

Another aspect of the invention relates to a method for corona treatingthe inside surface of a flexible thermoplastic tubular film wherein asubstantially flat film tube is provided with particles of less thanabout 11/2 mil (37.5 microns) average size within the inner space of theflat tube. The particles are provided in sufficient quantity to separateat least a portion of opposite inside surfaces of the flat tube in thetransverse direction, and provide voids between the particles within theflat tube. An electric discharge is established across these oppositeinside surfaces through the voids of the particle-containing flat tubeso as to corona treat the inside surfaces.

The inside surface to be corona treated is preferably a polyolefin andmost preferably contains EVA. The particles are preferably starch andalso preferably at least a portion of the particles are adhered to thetube inside surface prior to the electric discharge. The initiallyunadhered particles are adhered to the tube inside surface during theelectric discharge. Also, after the corona treatment the particles arepreferably present within the inner space of the flat tube in sufficientquantity to prevent the tube inside surfaces from blocking. When theparticles are starch they are preferably present in quantity of betweenabout 20 and about 30 gms/1000 ft² tube inside surface. Gas ispreferably introduced in the particle-containing flat tube inner spacein sufficient quantity to further separate the particles withoutinflating the tube.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a food product package including a foodproduct encased in a tubular cook-in film;

FIG. 2 is a view of an enlarged scale showing in cross section a portionof a three layer tubular wall embodiment of the package of FIG. 1;

FIG. 3 is a schematic drawing of process steps suitable for coronatreatment of powdered starch-containing irradiated tubular film; and

FIG. 4 is an enlarged detailed schematic drawing of the FIG. 3 coronatreatment.

DETAILED DISCUSSION OF THE INVENTION

The prior art has known that wetting of the film surface by the foodproduct causes the film to adhere to the food product encased and cookedin the film. It now has been found that the wetting of the film surfaceand food adhesion of an EVA-containing film surface are enhanced by bothirradiating the film and corona treating the film surface which becomesthe food adhering surface. The sequence of corona treating the film'sfood adhering EVA-containing surface and irradiating at least the filmthickness containing this EVA, is not critical. As will be demonstratedin an example, tests have shown that reduced cook-out (purge) isrealized irrespective of whether the EVA-containing surface is firstirradiated or first corona treated, as long as both operations areperformed on the film to be used in preparation of the package used tocook the food.

While irradiation and corona treatment are each known to improve themeat adherence of films, the improvement derived from the combination ofirradiation and corona treatment of the food adhering surface issurprising and unexpected. This is because the wetting tension of thefilm surface is one factor which determines whether the food product isable to wet the film surface and therefore, the adhesion characteristicsof the film. If all else is equal, films having comparable wettingtension values should have comparable food adhesion characteristics.However, it has been found that, even though an EVA-containing filmsubjected only to corona treatment has about the same wetting tension asthe same film subject to both irradiation and corona treatment, thecook-out (purge) characteristic of the latter is substantially lower.

These film structures are preferably "heat-shrinkable", and as usedherein this means the film has an unrestrained shrinkage of at least 20%in both the machine and transverse directions at 90° C. Typical shrinkvalues for this type of film are in the range of 30-40% in bothdirections.

The films of this invention also preferably have a shrink force at 90°C. in grams/mil of at least 100 in both the machine and transversedirections, and typical shrink force values for the aforedescribed threelayer heat shrinkable films are about 125 in both directions. The shrinkforce is that force or stress required to prevent shrinkage of the film.A high shrink force insures that the film pulls the cooked meat togetherduring the cooking cycle so as to reduce the possibility of void spaceswhere purge may accumulate. Shrink force is determined by the followingprocedure: Film samples are cut 1 inch (2.54 cm) wide by 7 inches (17.8cm) long in the transverse direction. The thickness of the film sampleis determined and the film sample is secured between two clamps spaced10 cm. apart. One clamp is a fixed position and the other is connectedto a strain gauge transducer. The secured film is then immersed in asilicone oil bath maintained at a constant elevated temperature for aperiod of five seconds. During this time, the force in grams at theelevated temperature is recorded. At the end of this time, the filmsample is removed from the bath and allowed to cool to room temperaturewhereupon the force in grams at room temperature is also determined. Theshrink force for the film sample is then determined from the followingequation wherein the result is obtained in grams per mil of filmthickness (g/mil): Shrink Force (g/mil)=F/T wherein F is the force ingrams and T is the average thickness of the film samples in mils.

Certain terms as used herein are to be understood as having thefollowing meanings:

"Cook-in" refers to a film structurally capable of withstanding exposureto long and slow cooking conditions while containing a food product forin situ cooking, for example submersion in water at 70°-80° C. for 4-6hours, or cooking in steam. Cook-in packaged food such as meats oftenare pre-packaged, pre-cooked meats which are directly transferred to theretailer still encased in the cook-in film. These types of meats may beconsumed with or without warming. Cook-in films maintain integrity ofthe package and in the case of multilayer films, are delaminationresistant. Cook-in films are preferably (but not necessarily) heatshrinkable under cook-in conditions so as to form a tightly fittingretail package. As an alternative, a meat processor may process a meatproduct in a cook-in film and after processing, remove the film andrepackage the product, either whole or sliced, for sale at retail. Ineither instance, a cook-in film of the present invention must bewettable by the uncooked meat product so that the film will adhereduring cooking, thereby reducing cook-out (purge).

"Cook-out" or "purge" refers to a liquid component collected from a meatproduct package after processing in a cook-in film. Cook-out or purge isquantified as a percentage of the total cooked food product packageweight.

"Food product" means edible food as for example meat.

"Irradiation" means exposure to high energy radiation such as electrons,X-rays, gamma rays, beta rays and the like, which induce cross-linkingbetween the molecules of an irradiated polymer. Preferably, irradiationis carried out by an electron accelerator and the dosage level isdetermined by standard dosimetry methods. The dosage is measured in"rads" wherein one rad is the absorbed dose of ionizing radiation equalto an energy of 100 ergs per gram of irradiated material. A megarad (MR)is one million rads.

"Adhering relation" between the aqueous medium cooked food outer surfaceand the film inner surface of the cooked food product package means thatbased on a peel test with a universal testing machine such as theInstron Model TM (Instron Corporation, Canton, Mass.) at a pull rate of10 in./minute, the film will not separate from the meat at a force belowabout 10 grams for a one inch wide strip of film.

"Wetting tension" refers to a measure of the surface energy of a film inaccordance with a test as described in ASTM D2578-67.

This invention also includes a cook-in film comprising:

a) a layer having a surface adapted for direct meat contact and havingat least 25 wt. % ethylene vinyl acetate, starch particles on saidsurface in an amount of at least about 20 gm/1000 ft², and

b) the surface being irradiated at a dosage of at least about 2 MR andcorona treated at an energy level sufficient to provide the surface witha wetting tension of between about 36 and about 50 dyne/cm. This cook-infilm is preferably a biaxially heat shrinkable multilayer tubecomprising an oxygen barrier core layer between inner and outer tubularlayers. The inner tubular layer has an inside surface defining the meatcontact surface, the inner and outer tubular layers both containethylene vinyl acetate and the film has shrink force of at least about100 g/mil at 90° C. both the machine and transverse directions.

Referring to the drawings, FIG. 1 shows a food product package generallyindicated at 2. The package includes a food product 3 encased in atubular cook-in biaxially heat shrinkable film casing 4. As shown inFIG. 1 the tubular casing is clipped at its ends 5, 6 to encase the foodproduct. As an alternative one or both ends of the tubular casing may beclosed by a heat seal (not shown).

FIG. 2 illustrates in more detail the structure of the foodproduct-encasing film. In this regard the casing 4 is a cook-in tubularfilm as may be used in accordance with the present invention. The filmpreferably is a three layer film of the general type disclosed in U.S.Pat. No. 4,784,863 and U.S. Pat. No. 5,051,266, the disclosures of whichare incorporated herein by reference. Briefly, FIG. 2 shows a preferredfilm to comprise a three layer tubular film including an outer layer 14,a core layer 16 and an inner layer 18.

The outer layer 14 is considered the abuse or abrasion resistant layer.The core layer 16 is a barrier layer, which functions with the otherlayers to provide an oxygen transmission rate through the entiremultilayer film of below about 5 cc/100 in.² /24 hrs./Atm. Inner layer18 has the inner surface 20 of the tubular film which is against a meatproduct encased and processed, eg. cooked, within the tubular film.

In the film of the present invention the outer layer 14 preferablycontains an ethylene vinyl acetate with a vinyl acetate content in therange of between about 3% and about 18%. This range provides a preferredfilm with the preferred biaxial heat shrink properties and adhesion tothe core layer and allows the needed biaxial orientation and cook-inperformance. The melt index of the EVA outer layer is preferably betweenabout 0.1 and about 1.0 to facilitate extrusion.

Alternatively, the outer layer may be formed of other thermoplasticmaterials as for example polypropylene, ethylene-propylene copolymer,ionomer or a member of the polyethylene family such as linear lowdensity polyethylene (LLDPE), very low density polyethylene (VLDPE), orblends of these materials. VLDPE is also called "ultra low densitypolyethylene" and comprises copolymers of ethylene with alpha-olefins,usually 1-butene, 1-hexene or 1-octene, and in some instancesterpolymers, as for example of ethylene, 1-butene and 1-hexene. Thedensities of VLDPEs are recognized by those skilled in the art to rangebetween about 0.86 and about 0.914 g/cm³. In a preferred film, the outerlayer is a blend comprising 75 wt. % VLDPE and 25 wt. % EVA.

The outer layer thickness is preferably between about 0.5 and 1.0 mils.Thinner outer layers may be less effective in performing the abuseresistance protection, while thicker outer layers may reduce heatshrinkability of the multilayer film.

As noted above, the core layer 16 functions to limit oxygen transmissionthrough the film. This is necessary to avoid spoilage of certain foodsenclosed in the cook-in film package due to oxygen passage from theenvironment through the film wall. This requirement is more importantwhere the encasing cook-in film is not removed for retail sales and maybe satisfied by numerous well-known barrier layer materials. Examples ofsuitable materials include certain of the polyamides (nylon), hydrolyzedethylene vinyl acetate copolymer (EVOH) and preferably a vinylidenecopolymer, most preferably a vinylidene chloride-methyl acrylatecopolymer i.e. MA-VDC. An MA-VDC type barrier layer is preferred becausethe oxygen barrier property is not affected by moisture, adhesive layersare not required and discoloration during cooking is minimal.

Inner layer 18 of the film has the film surface 20 in direct adhesivecontact with the cooked food product. This inner layer preferably has athickness of between about 0.5 and about 2.0 mils. Thinner layers maynot be adequate to perform the intended functions of this layer whilethicker layers may reduce total film performance.

The film inner layer contains an EVA preferably having a VA content ofbetween about 3% and about 18% by weight VA. A VA content in this rangeprovides the preferred biaxial orientation needed for heatshrinkability, adhesion to the core layer 16, and also the cook-out(purge) and food adhesion needed for improved cook-in performance. Ahigher VA content tends to make the film excessively soft and notcapable of the biaxial orientation needed for production of a shrinkfilm. The melt index of the ethylene vinyl acetate inner layer ispreferably between about 0.1 and 1.0. Lower melt indexes are notpreferred because the resulting high viscosities make extrusion verydifficult, whereas higher melt indexes are preferably not used becauseorientation strength is diminished.

The food adhering surface of the present cook-in films may have EVA asthe major constituent or may be substantially completely EVA or maycomprise a blend of EVA and another compatible thermopolymer material asfor example an unneutralized copolymer of the ethylene acrylic acid(EAA) type as described in the aforementioned U.S. Pat. No. 5,021,266.In particular, a suitable blend is in the range of 30-75 wt. % of EVAand 25-70 wt. % EAA, preferably 60% EVA and 40% EAA. As anotheralternative, EVA may be present as a minor constituent as described in asubsequent example wherein the food adhering surface is 25 wt. % EVA and75% VLDPE. In this instance, the VLDPE broadens the heat sealing rangefor the food adhering surface and this may be desirable where thissurface is heat sealed to itself as the inner layer of a food package inthe tubular form.

The multilayer film preferably as used in the food product package ofthe present invention could be formed as a flat sheet using a slot dieand tentering to achieve biaxial orientation. Preferably, the film ismanufactured in tubular form by simultaneous coextrusion of the threelayers using a double bubble biorientation technique as disclosed forexample in Pahlke, U.S. Pat. No. 3,456,044. In this technique anextruded primary tube leaving the tubular extrusion die is cooled,collapsed, and then preferably oriented by reheating and reinflating toform a secondary bubble. The film preferably is biaxially orientedwherein transverse orientation (TD) is accomplished by inflation toradially expand the heated film and machine direction orientation (MD)is accomplished with the use of rolls rotating at different speeds topull or draw the film tube in the machine direction. The biaxiallyoriented tube is cooled, flattened and guided through an ionizingradiation field at a dosage of at least about 2 MR.

The stretch ratio in the biaxial orientation is preferably sufficient toprovide a multilayer film with total thickness of between about 1.5 and3.5 mils. The MD stretch ratio is typically 3-5 and the TD stretch ratiois typically 3-5. A stretch ratio (MD stretch multiplied by TD stretch)of about 9-25:1 is suitable.

The preferred biaxially oriented heat shrinkable multilayer film ispreferably wound up as flattened, seamless, tubular film. The tubularfilm may then be formed into bags by end seals, typically made by clipsor by transverse heat sealing across the width of flattened tubingfollowed by severing the tubing so that the transverse seal forms thebag bottom. Alternatively side-seal bags may be formed in which thetransverse seals form the bags sides and one edge of the tubing formsthe bag bottom, or the tubular stock may be slit into sheet stock forfurther forming into back-seamed tubes by forming an overlap or fin-typeseal. In still another embodiment the tubes may be shirred and thenstuffed with a food product using a conventional stuffing machine suchas a Viskase Corporation SHIRMATIC® 600 Sizer which applies clips toseal both ends of the stuffed article.

One aspect of the cook-in films disclosed by both the '266 and '863Patents is that the film surface in contact with the food product shouldcontain starch particles uniformly dispersed over the surface at therate of at least about 20 gm/1000 ft². The presence of these starchparticles not only prevents blocking when the irradiated tubular film isin a laid flat condition, but the particles improve the meat adheringcharacteristics of the irradiated EVA and EVA/EAA inner layers. Forexample, U.S. Pat. No. 4,784,863 discloses that irradiated starchparticles on a meat adhering film surface formed of EVA is essential foracceptable meat adhesion. However, U.S. Pat. No. 5,051,266 disclosesthat acceptable meat adhesion is obtained without the use of starch ifthe meat adhering film surface formed of EVA includes at least 25% byweight EAA.

In the present invention, small particles preferably formed of starch ofabout 22-24 micron average size formed of starch are preferably appliedto the film inside surface before irradiation and corona treatment, at arate of at least 20 gm/1000 ft² and preferably between about 25 andabout 30 gm/1000 ft². These starch particles prevent blocking. They alsocreate voids or air gaps between the film inner surfaces in which thecorona discharge occurs, thereby enhancing performance by reducingcook-out (purge). The starch particles may be applied to theEVA-containing surface while the latter is hot as during extrusion, butmay alternatively or additionally be applied after extrusion when thefilm inner surface is cold. Reference is made to both U.S. Pat. No.4,784,863 and U.S. Pat. No. 5,051,266, the disclosures of which areincorporated herein by reference, for a more detailed description of thestarch application.

If the film is to be printed, the starch particles are preferablyapplied in two loadings, initially during extrusion up to aconcentration of about 12 gm/1000 ft². At least the initial loading ofstarch particles is preferably irradiated for improved fat-out (purge)and higher food adhesion. The film is then printed and after printingthe remaining starch is applied.

In accordance with one embodiment of the present invention, the foodadhering surface of the irradiated tubular cook-in film is subjected toa corona treatment to provide enhanced adhesion to the food productcooked in the film. In this respect, FIGS. 3 and 4 illustrate inschematic fashion the corona treating operation used for the filmdescribed in the following examples.

FIG. 3 shows an irradiated tubular film 22 with irradiated starchparticles on its inside surfaces, which is wound off supply roll 23 as aflattened tube, over tension control dancer roll 24 and then overupstream roller 25 making a 90° wrap. Sufficient pressurized air isperiodically introduced through flexible hose 25a to separate the tubeinner surfaces and form reserve air bubble 26 maintained by and betweenupstream roller 25 and downstream roller 27. The film is thereby wrappedabout 90° to minimize air transport to the corona treatment system. Theair introduction may for example be when a new roll 23 is positioned andthreaded through the system. A small quantity of air is carried throughdownstream nip roller 27 entrapped in the voids between starch particlescaptured between the opposite inside surfaces of the tube 28 as it isconsecutively moved in a loose "S" wrap configuration by rollers 27 and29 and through intermediate roller 30 respectively to contact withelectrically grounded rotating drum 31. The purpose of this restrictedair introduction is to further separate the starch particles within theflat tube, prevent agglomeration and increase the void space betweenparticles within the flat tube.

As shown more clearly in detailed schematic FIG. 4, multiple electrodes32 are circumferentially spaced from and around a portion ofcircumference of rotating drum 31. Starch particles 33 adhered toopposing inside surfaces of flat film 28 provide voids 34 which containentrapped air carried over from reserve air bubble 26, but the starchparticles are the sole means of separating opposite inner surfaces ofthe film. A corona charge 35 is applied by electrodes 32 across tube 28to grounded rotating drum 31 in accordance with the corona treatmentmethod invention. This is facilitated by the voids 34 within tube 28which exist among the starch particles providing pathways for dischargeof the electric arc between the tube inside surfaces. In this particularcovered roll-metal electrode system purchased from Pillar Company asModel AB 1326-1A (Hartland, Wis.), drum 31 is 12 inches diameter and hasfive electrodes spaced 4 inches center-to-center. The applied wattage isabout 1870, comprising about 9.7 amperes and about 193 volts. It hasbeen determined that for the small scale system used in the examples ofthis specification, a starch powder loading of 20-30 gms/1000 ft² filmsurface is necessary to provide the void volume needed to establish andmaintain the corona discharge. The air introduced through removable hose25 is periodically replenished, but is not present in sufficientquantity to actually inflate tube 28. The air does open the flat tubewhich on occasion may be slightly blocked. By way of illustration, about0.5 cu. ft. STP air is introduced and consumed during corona treatmentof 3,000 ft. of 81/2 inch flat width tubular film.

In this particular equipment, the major portion of the outside surfacecorona discharge is formed between the electrodes 32 and the top layer36 of tube 28. A sufficient corona also forms inside the film tube 28because of the aforedescribed void space among starch particles. Anothersmaller corona also occurs between the bottom layer 37 of tube 28 andthe covered treated roll 31. It has been observed that when there arenumerous unadhered starch particles in the air-containing casing 26because of high loading, eg. 30 gm/1000 ft², virtually all of thesestarch particles are adhered to the EVA-containing inside surface duringcorona treatment.

It should be noted that the FIG. 4 corona discharge zone of this systemdoes not require power-driven nip rolls for moving starch-containingflat tube 28 through this zone. Since the tube is flat and not inflated,there is no need for the tube to be isolated from the rest of the system(as in the prior art corona treatment systems) to maintain the tube inthe inflated condition. The voids 34 between particles 33 facilitateelectric discharge from fixed-position electrodes 32 to groundedrotating drum 31. More particularly, the flattened tube 28 is moved in aloose "S" wrap configuration by rollers 27 and 29 with gas to increasethe void space needed to facilitate the corona discharge. The fixedposition electrodes 32 are of course much less expensive to manufacturethan the conventional slip ring-containing roller electrodes employed bythe prior art.

Another difference between the FIG. 4 corona discharge system and theaforedescribed prior art system is the structure and position of spacedposition electrodes 34. They are fixed in position (except forlongitudinal adjustment and then do not require the conventional slipring. They are much simpler in construction and much lower inmaintenance requirements than the conventional roller-type electrodes.Since they do not support flattened tube 28 in the corona dischargezone, they may be longitudinally spaced from the tube top layer outsidesurface 36 by air gap 39. This facilitates the aforementioned coronadischarge 35 on the top surface 36 in addition to the tube inner surface37. Corona treatment of the flat tube top layer outside surface 36 maybe desirable to improve ink adhesion during one-sided printing.

In the corona treatment method aspect of this invention, air is used tofill the voids between adjacent particles within the flat tube, butother gases are usable as for example nitrogen, carbon dioxide, or othergases.

Whereas starch particles are preferred as the inner layer separatingmeans other types of particles less than about 1.5 mil (37 microns)average size may be used, as for example silica powder. If the insidesurface corona treated film is to be used for packaging food, as forexample in the aforedescribed cook-in system to produce a cooked foodproduct package, the particles are in direct contact with food andshould be approved by the appropriate governmental agencies for thisuse.

Also, in this corona treatment method the particles may have alreadybeen adhered to the tube 28 inside surfaces before corona treatment, asdescribed in connection with FIG. 3. Alternatively a portion of theparticles are adhered to the tube inside surface prior to the gasintroduction and the initially unadhered particles are preferablyadhered to the tube inside surface during the electric discharge.

With respect to the quantity of particles required in the coronadischarge method of this invention, they must be sufficient in quantityto separate at least a portion of opposite inside surfaces of the flattube in the tube transverse direction (shown as dashed line T--T in FIG.4) to provide voids between the particles within the flat tube. Thevoids must in turn be of sufficient volume so that an electric dischargepathway may be established across the opposite inside surfaces of thenon-inflated tube through the voids. In the experiments comprising theworking examples described herewithin, it was determined that at leastabout 20 gms starch/1000 ft² tube inside surface is needed to establishand maintain the electric discharge and realize the desired coronatreatment. A preferred range is between about 20 and about 30 gms starchparticles/1000 ft² tube inside surface. This quantity of starchparticles is also sufficient to prevent the tube inside surfaces fromblocking after the corona treatment.

One difference between the cooked meat product package aspect of thisinvention and the corona treatment method aspect of the invention isthat the inner surface of the thermoplastic tubular film used in themethod need not, but preferably contains ethylene vinyl acetate.However, this inner surface is preferably a polyolefin as for examplehomopolymers of olefins, copolymers of olefins (including terpolymers),and copolymers of the olefin and other monomer copolymerizable therewithsuch as other vinyl monomers. Specific examples of such polyolefinssuitable for use as the inner surface to be corona treated includepolyethylene having a very low density to high density, polypropylene,polybutene, copolymers thereof, ethylene-acrylic acid copolymers, andmodified polyolefin resins. Typical examples of the modified polyolefinresins include modified polymers prepared by copolymerizing, e.g. graftcopolymerizing, the homopolymer of the olefin or copolymer thereof withan unsaturated carboxylic acid, e.g. maleic acid, fumaric acid or thelike, or a derivative thereof such as anhydride, ester or metal salt orthe like. These modified polymers are useable singly or in mixture witheach other or with other resin components, e.g. other polyolefin resins.

Although the film inner surface used in the method aspect of theinvention is preferably formed of polyolefin, other thermoplasticmaterials can be used, as for example polyesters, polystyrene, polyvinylchloride and polyamides.

The film to be corona treated by the method of this invention may be amonolayer, with the material of the inner surface comprising the entirefilm. Alternatively it may be a multilayer film as for example the typeuseful as the previously described preferred cook-in film comprisingthree layers. In this embodiment the inner layer is the layer whoseinside surface will be corona treated, the core layer is the oxygenbarrier, and the outer layer provides abuse resistance.

The to be corona treated film may be manufactured in the same manner asthe previously described cook-in film. Unlike the latter, in the coronatreatment method of this invention the film is not necessarilyirradiated. Moreover it need not necessarily be biaxially oriented asthis will depend on the end use.

Notwithstanding the aforedescribed advantages of the corona treatmentmethod of this invention, the cook-in film used to prepare the cookedfood product of this invention may be corona treated in the conventionalmanner instead of the FIGS. 3-4 system. More particularly the coronatreatment system may be placed between horizontally spaced nip rollers.Inflation air would be introduced between the spaced nip rollers to forma stable bubble. A multiplicity of roller-type electrodes would belongitudinally spaced in pairs on opposite sides of the film bubble,with one dielectric unit of each pair attached to the power supply andthe other bare metal unit being grounded. It is believed thatnotwithstanding the metered air gap between electrodes--about 0.06 inch,it would still be desirable to use small particles eg. starch, toprevent blocking of the corona treated EVA-containing inside surfaces.

Returning now to FIG. 3, the irradiated and corona treated film tube 40is guided off rotary drum 31 by guide roller 41 and then wound ontotake-up roll 42. If needed the external surface of tube 40 may be firstcontacted by brushes 43 to wipe off corona treatment occurring on thetube external surface and thus avoid the possibility of externalblocking of the irradiated and corona treated film in roll 42.

Internal blocking may be a concern if this irradiated and corona treatedfilm is stored in roll form for a sustained period before consumption,particularly if the roll is relatively large in diameter, eg. at least16 inches, and/or the roll is stored at high temperatures. In thisevent, internal blocking probably could be avoided by the use ofadditional internal and external starch particles, looser rewindtension, cool storage or other means to minimize roll hardness.Alternatively, the film could be formed into shirred sticks, i.e.longitudinally compressed into pleats, as is well understood by thoseskilled in the art. It is believed that in general, the compressiveforces on the tube inner surfaces are substantially lower in the shirredstick form than in the roll form, so the blocking tendency is greatlydiminished by shirring and storing the irradiated and corona-treatedfilm tube as sticks rather than in roll form.

The purpose of corona treatment is to increase the wetting tension orsurface energy of the meat contacting film surface. For purposes ofpurge reduction, a high wetting tension and good adhesion of the film tothe meat product is desired. The intensity of the corona treatmentdepends in part on the speed of the film over the rotating, groundeddrum 31, and on the density of the electrical discharge. For purposes ofthe present invention, corona treatment is preferably sufficient toincrease the surface wetting tension of an EVA-containing film to atleast about 36 dyne/cm. The surface wetting tension is preferably lessthan about 50 dyne/cm so that adhesion is not so great as to causedisruption of the cooked meat surface when the film is removed.Excessively high adhesion could result in chunks of cooked meatremaining on the film surface. Those skilled in the art will recognizethat the meat's resistance to surface disruption depends to some extenton its own internal cohesive forces. For example, high protein meatshave greater resistance to surface disruption than meats with high fator high water content.

Embodiments of both main aspects of the present invention (including thecooked food product package, the cook-in film and the corona treatmentmethod) are illustrated by the following examples. In these examples thefilms used as the controls are commercial cook-in films. In particular,one commercial film is a three layer cook-in tubular film of the typedescribed in U.S. Pat. No. 4,784,863. The core layer of this film is 75%vinylidene chloride-methyl acrylate, 25% vinylidene chloride-vinylchloride and its outer layer is a 75/25 weight ratio VLDPE/EVA blend.The meat adhering inner layer of this tubular film is composed of EVAhaving a VA content of about 10 wt. %. Another commercial film, also athree layer cook-in tubular film, is of the type disclosed in U.S. Pat.No. 5,051,266. This film also has a MA-VDC core layer and a 75/25VLDPE/EVA blend in the outer layer. However, the meat adhering layer ofthis film is composed of a 65/35 weight ratio blend of EVA and EAAwherein the VA content of the EVA is about 10 wt. %. Commercial films ofboth types have starch particles uniformly distributed over the meatcontacting surface at a dosage rate of about 20 g/1000 ft² and each filmafter starch application is subjected to an irradiation of about 4 MR.

EXAMPLE I

A commercial three layer film as disclosed in U.S. Pat. No. 5,051,266was used as a control. This film as noted above is a tubular film havinga MA-VDC core layer and an inner layer composed of the aforedescribedblend of EVA and EAA. The outer layer of the tubular film is a blend of75% VLDPE (density 0.912) and 25% EVA (10% VA). The film has starchparticles applied to its inner surface during extrusion at a level ofabout 20 gm/1000 ft² and then the film is irradiated (externally) at adose of about 4 MR.

Samples of the same three layer film structure were irradiated at adosage level of about 4 MR and then these samples were internally coronatreated as described above. Instead of the 12 gm/1000 ft² starch usedwith the prior art films, a higher level of 20 to 30 gm/1000 ft² wasapplied to the EVA-containing inner layer during extrusion and prior toirradiation. This was preferred to avoid blocking between the tube innersurfaces which because of the present invention are more adhesive thanthe corresponding prior art surfaces.

It was found that the internal surface of the control film had a wettingtension of about 34 dyne/cm as determined by ASTM Test Method D2578-67.By the same test method, the wetting tension of the internal surface ofthe corona treated samples was about 39 dyne/cm. Also, in the particularmethod used to apply a corona discharge on the tube inner surface in theworking examples of the specification, this level of starch loading wasnecessary to provide the voids to excite an ionizing discharge insidethe tube.

All films were stuffed with a chunked ham formulation wherein the targetstuffed circumference was 1.07 times the measured unstuffedcircumference to insure that the film was taut about the meat product.After stuffing, the meat product packages were cooked with steam untilthe package reached an internal temperature of 155° F. and thereafterthe packages were showered with tap water for about 40 minutes.

Evaluation involved weighing the package after cooking and thenstripping the film from the cooked meat product. All the exudate isremoved from both the stripped film and the meat surface and the filmand cooked meat are then reweighed. The cook-out weight loss or "purge"represented by the removed exudate is expressed as a percentage of thecooked meat product package weight. The results of these tests are setout in Table I.

Also, the adhesion between the film inner surface and the meat surfacefor each package was evaluated using an Instron machine mentionedhereinabove. In the test procedure a one inch wide by six inch long teststrip was slit in the meat product package. The package was placed on ahorizontal plane and the one inch slit end of the strip was attached tothe load cell of the Instron machine. The test was initiated by loweringthe plane at a rate of about 10 inches per minute and the force requiredto pull the six inch test strip of film from the encased meat wasrecorded. This force varies as increments of the strip are pulled fromthe meat surface and the average force to peel is noted in Table Ibelow.

The results of adhesion and purge tests, all conducted as notedhereinabove, are reported in Table I. Each sample is the averagedperformance of two cooked meat product packages.

                                      TABLE I                                     __________________________________________________________________________    Chunked Ham Adhesion                                                                       Wetting     Target  Stuffed Adhesion By                              Irradiated                                                                             Tension                                                                             Flatwidth                                                                           Circumference                                                                         Circumference                                                                         Instron Test                                                                         Purge                         Sample                                                                            Film Type                                                                              (dyne/cm)                                                                           (inches)                                                                            (inches)                                                                              (inches)                                                                              (gm/inch)                                                                            (%)                           __________________________________________________________________________    A   (Control)                                                                              34    7.00  15.00   15.06   73     1.00                          B   (Corona treated)                                                                       39    7.88  16.80   15.80   109.7  0.30                          C   (Corona treated)                                                                       39    8.0   17.10   17.2    109.7  0.30                          __________________________________________________________________________

The results as shown in Table I demonstrate that a cooked meat productpackage having an irradiated and corona treated meat adheringEVA-containing inner surface (samples B and C) provides enhanced purgeand meat adhesion characteristics over the control package (sample A)including the same irradiated but not corona treated, EVA inner surface.

EXAMPLE II

In a second test, three commercial films of the type disclosed in U.S.Pat. No. 4,784,863 were compared to determine the relative effects ofirradiation and corona treatment on the meat adhering properties of thefilm.

This film was a three layer biaxially oriented heat shrinkable tubularfilm of 11 inches flat width having an MA-VDC core layer, a 100% EVAinner layer and an outer layer composed of the same VLDPE-EVA blend asthe film of Example 1. During extrusion of films for this test, starchparticles were applied to the inner surface of the film at a rate ofabout 12 g/1000 ft². In an off-line process, additional starch wasapplied to the film inner surface at a level of about 18 g/1000 ft²bringing the total level of powdered starch up to about 30 g/1000 ft².This base film was used to produce each of the test samples as notedbelow.

For Sample D, the film was laid flat and irradiated to a level of about4 MR.

For Sample E, the film was not irradiated but the inner surface of thetubular film was subject to a corona treatment in a manner as describedhereinabove, and in accordance with the corona treatment method of thisinvention.

For Sample F, the film was laid flat and irradiated to a level of about4 MR. After irradiation the inner surface of the tubular film Sample Fwas corona treated in accordance with the corona treatment method ofthis invention.

The wetting tension of each film was tested using the same ASTM TestMethod D-2578-67. The tubular films were then each stuffed with achunked and formed ham product wherein the target stuffed circumferencefor each piece was 1.07 times the measured circumference of theunstuffed film. All meat product packages were then cooked with steam toan internal temperature of 155° F.

After cooling, the cooked product packages were tested for adhesion andpurge in the same manner as previously described. The results of thesetests, together with the wetting tension of each film sample, is set outin Table II. Each sample is the average performance of two cooked meatproduct packages.

                  TABLE II                                                        ______________________________________                                        Corona Treating and Irradiation Alone and Together                                             Inside                                                                        Corona  Wetting                                                               Treat-  Tension Adhesion                                                                              Purge                                Sample Irrad     ment    (dynes/cm)                                                                            (gm/inch)                                                                             (%)                                  ______________________________________                                        D      Yes (4MR) No      32      37.9    1.30                                 E      No        Yes     38      51.7    0.57                                 F      Yes (4MR) Yes     39      60.3    0.24                                 ______________________________________                                    

Examination of the test results as reported in Table II confirm that thefilms having the greatest meat adhesion had lowest purges andconsequently will provide the highest yields after cooking.

However, a further and surprising result of these tests is seen in thecomparison of Samples E and F. In this regard the results show that bothcorona treated Sample E and F had about the same wetting tension of38-39 dyne/cm. Accordingly, it would be expected that both samples wouldhave comparable adhesion and purge characteristics. However, andsurprisingly, the irradiated and corona treated film Sample F had higheradhesion and less purge than the nonirradiated but corona treated SampleE.

EXAMPLE III

Another series of tests was run to compare the sequence of irradiationand corona treatment of EVA-containing films as affecting the purgereduction and improved meat adhesion achievable: with this invention.

The films used in the Example II tests were also used in this series,and the cook-in procedure was identical. Sample G was identical toSample F except that it was used to package high water content (48%)chunk and form ham emulsion about one month following completion of thefirst irradiation--then corona treatment sequence, and was used in theExample III cook-in tests about two months after the Example II tests.Sample H was the original corona treatment-only Sample E, which was thenirradiated at 4 MR about one month later. Two cook-in meat productpackages were prepared comprising each sample. The data is summarized inTable III, and shows that the adhesion and purge values for Samples Gand H are very similar. This demonstrates that the sequence ofirradiation and corona treatment is not critical to realizing theadvantages of the invention, which are fully accomplished irrespectiveof which step is performed first. The Sample G data demonstrates that ifthis particular EVA-containing surface (comprising 100% EVA) issubsequently corona treated, the synergistic combined effectiveness doesnot rapidly deteriorate over a period of at least two months.

                  TABLE III                                                       ______________________________________                                        Corona Treating-Irradiation Sequence                                                               Wetting                                                                       Tension   Adhesion                                                                              Purge                                  Sample                                                                              Sequence       (dynes/cm)                                                                              (gm/inch)                                                                             (%)                                    ______________________________________                                        G     1st Irradiation                                                                              38        24.5    0.31                                         2nd Corona Treatment                                                    H     1st Corona Treatment                                                                         37        22.7    0.38                                         2nd Irradiation                                                         ______________________________________                                    

As part of the Example III tests, another identical sample of theEVA-containing film with about 30 gms/1000 ft² starch was irradiated at4 MR and used to enclose the same ham emulsion followed by the samecook-in treatment. This sample (without corona treatment) apparentlyprovided about the same adhesion (25.0 gms/in) and purge (0.33%) as theinvention Samples G and H, but this result was inconsistent with allother irradiation--only tests as example, Sample D. This discrepancy isbelieved due to mix up of samples, and Table II is believed to be aqualitatively and quantitatively correct comparison.

As still another part of the Example III tests, the film of Sample E(within only corona treatment and no irradiation) was retested about twomonths after the original corona treatment. The resulting cook-in meatproduct package provided much higher purge (3.95%) and poorer adhesion(1.1 gms/in). These results are very inferior to the original Sample Etest (0.57% purge and 51.7 gms/in adhesion), and suggest that withoutirradiation, the corona treatment decays much more quickly and loses itseffectiveness. This is in marked contrast to the previously discussedSample G performance relative to Sample H.

EXAMPLE IV

This series of tests was performed to demonstrate that theEVA-containing surface to be used for meat contact in accordance withthis invention need not comprise primarily EVA, nor is the presence ofstarch particles essential for improved cook-out (purge) and meatadhesion.

The tubular film (flat width about 11 inches) used in these tests wasidentical to the Example II film, i.e. MA-VDC core layer, a 100% EVAinner layer and a 75% VLDPE-25% EVA outer layer. Sample I was preparedfrom this film for comparison purposes After irradiation at 4 MR, thisfilm was corona treated on its outside surface and the wetting tensionwas determined to be 37 dyne/cm. The corona-treated tube was turnedinside out so that the corona treated surface was on the inside (ExampleJ). Ten months later the same inverted film (Sample K) still had thesame wetting tension of 37 dyne/cm.

These tubular films were used to package the: same high water contentchunk and form ham emulsion used in Example III, and hot water-cooked inthe same manner. The results of these tests are summarized in Table IV,and each sample represents the average of two packages.

                                      TABLE IV                                    __________________________________________________________________________    Low EVA Content Surface                                                                           Wetting                                                        Corona                                                                              Inside   Tension                                                                             Purge                                                                              Adhesion                                       Sample                                                                             Treatment                                                                           Surface  (dyne/cm)                                                                           (%)  (gm/inch)                                      __________________________________________________________________________    I    No    100%                                                                              EVA  34    1.55 35.4                                           J    Yes   75% VLDPE                                                                              37    0.09 137.1                                          K    Yes   75% VLDPE                                                                              37    0.32 24.5                                           L    No    75% VLDPE                                                                              32    0.33 23.6                                           __________________________________________________________________________

Table IV confirms that improved cook-out (purge) and meat adhesion isobtained when the meat contacting surface contains only 25% EVA, thebalance being VLDPE. More specifically, Samples J and K (25% EVA--75%VLDPE meat contacting surfaces which had been irradiated and then coronatreated) each displayed much lower purge values than Sample I (100% EVAmeat contacting surface which was only irradiated and not coronatreated). This improvement was obtained despite the fact that thewetting tensions were only slightly higher, i.e. 37 vs 34 dyne/cm.

Table IV also demonstrates that after a 10 month storage period, thepurge value undesirably rose and the adhesion diminished for theirradiated then corona treated 25% EVA--75% VLDPE meat contactingsurface. In particular Sample J provided purge and adhesion values of0.09% and 137.1 gm/inch respectively, whereas after 10 months the samefilm (Sample K) provided purge and adhesion values of 0.32% and 24.5gm/inch respectively. From this data it appears that the effectivenessof the 25% EVA--75% VLDPE meat contacting surface diminishes with time,and for best results the film should be used to form a cook-in meatproduct package reasonably soon after irradiation-corona treatment.

Sample L comprised an irradiated but not corona treated 25% EVA-75%VLDPE meat contacting surface formed by inverting the same tube used toform Samples J and K. The purge value (0.33%) was unexpectedly low andthe adhesion was similar to Sample K. This was an anomalous result andinconsistent with the performance of other EVA-containing irradiated butnot corona treated meat contact surfaces such as Sample A (Table I) andSample D (Table II). The latter are believed to accurately reflect thequalitative and quantitative performance of such surfaces.

It should be noted that invention embodiment Samples J and K did notinclude a coating of starch particles on the meat contacting surface,unlike the invention embodiments of Examples I-III. This was because thefilm tubes were prepared by inverting a tube of the type in commercialuse wherein the original inside surface was coated with starchparticles, but not the outside surface. Accordingly, Samples J and Kdemonstrate that the irradiated and corona treated EVA-containing meatcontact surface of this invention need not have irradiated starchparticles therein to achieve the improved cook-out (purge) and meatadhesion of this invention. However, small particles such as powderedstarch are preferably deposited to at least avoid blocking on theEVA-containing surface even if not needed to reduce cook-out and enhancemeat adhesion performance.

On the basis of these examples it is apparent that the combination ofirradiation and corona treatment of the EVA-containing inner layer ofthe cook-in films provide surprisingly enhanced meat adherence andcook-out (purge) characteristics over the corresponding prior art oncorona treated or irradiated films. Moreover, this enhancement ofadhesion and cook-out (purge) characteristics appear to be synergisticsince the enhancement occurs despite the fact that corona treated filmshave comparable wetting tension regardless of whether the films areirradiated.

While a preferred embodiment of the invention has been described, itshould be appreciated that changes and modifications may be made withoutdeparting from the spirit and scope of the invention. For example,although oxygen barrier type multilayer films have been described indetail, some cook-in applications may not require an oxygen barrier. Inthis event, the EVA-containing layer may comprise the entire film whichfor example may be prepared in the biaxially oriented heat shrinkableform by the aforedescribed double bubble process. As another variation,the inventive film need not be shrinkable but may be prepared as a blownor cast mono or multilayer film having little or no shrink. Such a filmmust rely on enhanced wetting tension for meat adherence. As a stillfurther variation, instead of the preferred starch particles adhered tothe EVA-containing surface, this function would be performed by small,eg. 30-35 micron particles of other materials suitable for directcontact with food, eg. silica.

We claim:
 1. A method for corona treating the inside surface of aflexible thermoplastic tubular film comprising the steps of providing asubstantially flat film tube with particles of less than about 1.5 milaverage size within the inner space of said flat tube in sufficientquantity to separate at least a portion of opposite inside surfaces ofsaid flat tube in the transverse direction and provide voids between theparticles within said flat tube; and establishing an electric dischargeacross said opposite inside surfaces through the voids of theparticle-containing flat tube so as to corona treat said insidesurfaces.
 2. A method according to claim 1 wherein said particles arestarch.
 3. A method according to claim 1 wherein said inside surface isa polyolefin.
 4. A method according to claim 3 wherein the polyolefininside surface contains ethylene vinyl acetate.
 5. A method according toclaim 1 wherein the particles are adhered to the tube inside surfaceprior to the electric discharge.
 6. A method according to claim 1wherein only a portion of said particles are adhered to the tube insidesurface prior to the electric discharge and the initially unadheredparticles are adhered to said tube inside surface during said electricdischarge.
 7. A method according to claim 1 wherein said particles arepresent within the inner space of said flat tube in sufficient quantityto prevent said tube inside surfaces from blocking after the coronatreatment.
 8. A method according to claim 1 wherein said particles arestarch and present in quantity of between about 20 and about 30 gms/1000ft² tube inside surface.
 9. A method according to claim 1 wherein priorto said electric discharge, gas is introduced in the particle-containingflat tube inner space in sufficient quantity to further separate saidparticles without inflating said tube.
 10. A method for corona treatingthe inside surface of a flexible thermoplastic tubular film comprisingthe steps of providing a substantially flat film tube with an ethylenevinyl acetate-containing inside surface and starch particles of lessthan about 1.5 mil average size within the inner space of said flat tubein quantity of between about 20 and about 30 gm/1000 ft² tube insidesurface so as to separate at least a portion of opposite inside surfacesof said flat tube in the transverse direction and provide voids betweenthe starch particles within said flat tube with at least a portion ofthe starch particles being adhered to the ethylene vinylacetate-containing inside surface; introducing gas in the starchparticle-containing flat tube inner space in sufficient quantity tofurther separate the particles without inflating said tube; andestablishing an electric discharge across said opposite inside surfacethrough the voids of the starch particle-containing flat tube so as tocorona treat said inside surfaces.